1. Field of the Invention
The present invention relates to a semi-rigid cable designed for the transmission of microwaves such as the waves used, for example, in radar or for cable television broadcasting, notably high definition digital television.
2. Description of the Prior Art
At present, the growing tendency with radar antennas is to make surface sampled active antennas which are, therefore, formed by a very great number of radiating and active elementary modules, each of these modules having an elementary transmitter and an elementary receiver which are proper to it. A radar antenna of this type, having dimensions generally of several meters, i.e. height and width, may comprise up to several thousands of these elementary transmitter/receiver modules.
For this type of antenna, it should be necessary to be able to convey the following by means of a number of microwave transmission cables at least equal to the number of modules on this network of transmitters and receivers:
firstly, the driving signals for the transmission;
secondly, the demodulation signals (the signal of the local oscillator for the reception function).
The signals picked up by these elementary receivers should be further capable of being conveyed in return to the processing installation, whether they are of the digital type or of the analog type.
This type of antenna therefore has to be fitted out with a network of cables for the distribution of microwave signals which may call for the use, in all, of several kilometers of cables of this type for one and the same antenna.
These transmission cables are subjected to severe constraints and requirements. They have to be totally free from mutual crosstalk. They should be perfectly shielded from the exterior, especially so that they are prevented from picking up the stray signals or from being sensitive to jamming or interference. Finally, in order to perfectly preserve the phase relationships of the illumination, there should be no phase noises: their phase should be stable and repetitive despite dismantling, transportation and re-assembly.
Cables meeting such requirements could also be necessary in the future to equip the high-definition digital television broadcasting networks. The pass-bands needed will be so high that it should be expected that there will be great difficulty in finding a sufficient number of radio channels without causing interference: it might subsequently be necessary to fall back on a cabled broadcasting system if it is desired to be able to obtain a satisfactory quality for the signals received by the users, as well as a sufficient broadcasting range.
The most widely used microwave cable is the braided metal-sheathed coaxial cable. This type of cable is not free of crosstalk unless it is double-sheathed. It thus has a central conductor with a circular section, surrounded by a dielectric which is itself surrounded by two superimposed, braided metal sheaths, with standard high-density braiding. The entire unit is, of course, covered with an external insulating sheath, capable of providing for mechanical protection and imperviousness to water. These standard cables are, however, not perfectly suitable, above a-1 because of contact noises, called "braid noises", which tend to appear in the two superimposed braids. These braid noises, which come into play especially during vibrations or other movements of the cable, are detrimental to the efficient transmission of the microwaves. Besides, the quality of the contacts between the two braided cables changes in the course of time, especially at the ends of the cable. Furthermore these cables are subjected to phase noises which are quite detrimental in certain cases. Finally, after a cable is deflected or undergoes a cycle of temperature, the two braided sheaths do not necessarily follow a return movement and, as a result, there are changes in characteristics. This standard braided double-sheathed cable is therefore, technically, not really satisfactory for the desired use.
The known approach, which is presently the most satisfactory one from the viewpoint of performance characteristics, consists in the use of semi-rigid coaxial cables formed by a solid, metal central core surrounded by a dielectric which is, generally, a dielectric with low losses such as polyethylene or PTFE which is itself coated with an external conductor made of solid copper.
This type of solid metal-sheathed coaxial cable is, however, ill-suited to fabrication by continuous wire-drawing process. The methods used for certain cable dimensions consist in the fabrication of this external conductor by wrapping a strip of sheet metal around its longitudinal axis so as to position it edge to edge, and then in seam welding these two edges. The cable is advantageously corrugated to facilitate the shaping operations.
Whatever the case may be, this cable is every costly and cannot be made on an industrial scale except in restricted lengths. It is therefore ill-suited to the above-mentioned applications, where it is necessary to implant very large quantities of cable lengths. It further has the drawback of requiring fairly costly connectors at each end. Finally, it is ill-suited to the multiple transmission of auxiliary service signals (telephone channels, signals reporting tests on distant equipment etc.).